1. Field of the Invention
The present invention generally pertains to a method and apparatus for aligning vehicle wheels, and more particularly, it pertains to a method and apparatus for measuring the correct individual toe angles of the front wheels of a vehicle.
2. Description of the Prior Art
It is necessary to the proper performance and also to the tire life of the front wheels of a motor vehicle that its front wheels be properly aligned. Accordingly, wheel alignment apparatus and methods have been devised to provide accurate measurement of the camber, caster, and toe angles of automotive vehicles, for example, to an accuracy .+-.1/64 in. or .+-.1/32.degree. for toe angles. Such measurements can and have been made with fairly simple instruments. However, in order to be effective, a wheel alignment system must be fast, accurate and convenient for the average mechanic, i.e., one having average skill and training, permitting its use for rapid and easy measurement during adjustment of the alignment parameters.
Before proceeding to summarize some problems of prior art systems, some alignment parameters will be briefly defined. The camber angle is recognized as the angle between the plane of a wheel and the vertical as seen from the front or rear of the vehicle. Caster angle is the angle between the steering axis of the wheel and the vertical as seen from the side of the vehicle. The angle of inclination of the steering axis is the angle between the steering axis and the vertical as seen from the front or rear of the car. As will be directly referred to in the description of the present invention, the individual toe angle, or toe, is the angle subtended between the plane of a front wheel and a vertical plane through the longitudinal centerline of the car, or, in the case where no reference to the longitudinal centerline is available, a vertical plane normal to the transverse line connecting the intersections of the wheel axis and the steering axis of each front wheel; and the total toe angle is the included angle between the planes of the two front wheels, which angle equals the sum of the individual toe angles. When the wheels have been properly aligned, the individual toe angles, i.e., right and left toe, will be equal and thus one half of the total toe. In addition, when the front wheels of the vehicle are properly aligned, the steering will be centered such that the vehicle will move straight ahead when the steering wheel is in its "dead center" position.
A typical opto-mechanical alignment system includes a pair of beam projector and target assemblies, each assembly being mounted to the outside of the rim of a front wheel such that the projector on each side directs a beam against the target of the assembly on the opposite wheel. Aligners of this sort are disclosed, for example, in U.S. Pat. Nos. 3,953,134 to Appel et al. and 3,393,455 to MacMillan. The targets or the beams include scales including lines indicative of particular angles or incremental distances which scales, for the particular vehicle involved, supposedly indicate the individual toe angle according to the location of the intersection of the beam of light projected from the opposite wheel with the target. No reference is made directly to the centerline of the car or indirectly to the centerline of the car by virtue of one or both of the rear wheels.
The foregoing alignment systems are referred to in the art as being of the cross-toe type. In brief, they measure the individual toe of a front wheel as a function of the linear translation of a beam of light from a projector mounted on the wheel with respect to a reference line in the plane of the target. It has long been noted that such systems are very inconvenient to use when gross misalignment problems are present, e.g., in cases of newly installed ball joints and other front-end suspension parts. A particular problem is that only a single measurement and adjustment of each wheel will often not yield satisfactory results when either or both of the initial individual toe angles are grossly incorrect. Even though the steering wheel is clamped so that the other wheel is undisturbed while making adjustment of the toe of one wheel, the toe reading of the other wheel will be seen to change as the adjustment is made. Thus, prior known systems often provide false individual toe readings indicating that adjustment is required for both wheels when, in fact, after consecutive adjustments it is found that one wheel needed no adjustment at all, i.e., the true value of individual toe was correct at the outset. This latter problem is vexatious when the wheel's adjustment involves the arduous, but unnecessary, task of loosening and retightening a rusted tie-rod.
More recently developed cross-toe systems utilize an electronic detection apparatus in lieu of the optical targets intended to be read directly by the operator. Some known prior art systems employing such apparatus are disclosed in U.S. Pat. Nos. 3,865,492 to Butler and 3,782,821 to Senften. Instead of a direct visual indication being provided on the target itself, the electronic systems use photosensitive elements at the target to generate signals dependent upon the angular position of light sources mounted on the wheel whose angular position is being determined. Such signals may be processed by logic circuitry to derive a further signal supposedly corresponding to the individual toe angle for the wheel. The electronic systems also include means for displaying the thus computed individual toe angles of the wheels. When the angle that is electronically displayed is that desired for the wheel being adjusted, the mechanic may then stop adjustment of that wheel and proceed to adjust the other wheel.
Even though electronic read-outs are provided, in any system in which a beam projector is mounted on one wheel directing a beam to a beam position detector on the other wheel, consecutive adjustments of the right and left front wheels still have to be made in the same manner as mentioned in connection with the discussion of the opto-mechanical wheel alignment systems. In practical terms this means that the mechanic must make an adjustment of one wheel until the electronic instrument displays the proper toe reading for that wheel. Then he will adjust the opposing wheel until its toe reading is correct or within specification. However, as soon as the second adjustment is made, it often happens that the previously adjusted toe angle for the first wheel will be incorrect, the likelihood of this happening being dependent upon the original amount of misalignment of the second wheel. The mechanic must then return to and bring the first wheel within specifications, and in so doing then find the second wheel to be incorrectly aligned. Another adjustment must then be made for the second wheel and then the alignment reading for the first wheel again checked.
It will be well appreciated that the required consecutive adjustments and readjustments as aforedescribed is physically taxing and time consuming. Furthermore, as previously indicated, it may well happen that at the outset only one wheel need be adjusted even though the initial readings of the alignment system indicate adjustment of both wheels is necessary.